Sometimes it is desirable to operate a generator at variable speeds rather than at a constant speed. Some applications where this is desirable is in generating electricity from wind power or hydropower. In these applications, the energy source is not constant. Therefore, it is desirable for generators used in such applications to operate at a speed and frequency which varies as the input energy source varies. However, the frequency of the output of the generator must be constant.
The equation which relates the various frequencies in a variable speed generator is: EQU f.sub.slip =f.sub.r -f.sub.s ( 1)
where f.sub.s is the synchronous or line frequency (typically 60 hertz); f.sub.r is the equivalent electrical rotor frequency and f.sub.slip is the slip frequency. The line frequency f.sub.s is the desired frequency or output frequency at which the generator must produce power. The equivalent electrical rotor frequency f.sub.r is related to the rotor speed. In a typical situation, f.sub.s is known and is the line frequency, and f.sub.r, although it varies with the rotor speed, can be determined from the rotor speed and the electrical configuration of the generator. Therefore, by varying the slip frequency f.sub.slip as the rotor speed varies, equation (1) can be satisfied enabling the variable speed generator to produce a constant frequency output at the line frequency f.sub.s.
Generally, the slip frequency in a variable speed generator is determined by a device known as a slip frequency generator. The slip frequency generator is part of the overall control system for the variable speed generator. Some examples of control systems which calculate or monitor the slip frequency are described in U.S. Pat. Nos. 3,371,306; 3,832,609; 4,327,420; and 4,463,306.
In previous control systems for variable speed generators, a closed loop analog or digital feedback system has been used to generate the slip frequency which should be applied to the variable speed generator to satisfy equation (1). In such systems, the output power of the motor was measured, calculations were performed by the circuitry and a new slip frequency was determined by the slip frequency generator which was then applied to the generator. The output power was again measured and further modifications and calculations were done. The problem with this type of control system was that the machine output was never exactly as desired. The control system was always chasing the desired behavior of the variable speed generator, especially if the input changed.
Another system for controlling a variable speed generator was an open loop system using a combination of analog and discrete digital circuitry to generate the appropriate slip frequency. This type of system required as its inputs, all three phases of the line voltage and the rotor speed. The inputs were processed by analog circuitry which included filtering, phase shifting, summation and multiplication. Digital logic was then used to select the appropriate slip frequency signal from the analog output. The disadvantage with this type of system was that the large number of analog and discrete digital circuit elements was subject to errors due to component drift over time and due to changes in temperature. The result was that the output of the control system could be significantly distorted. This resulted in an equivalent distortion in the generator output at the slip frequency and its harmonics.